Changing sprites for Dark Knight / Onion Knight

[darkskyx]

It's weird that after many years people didn't discover a way to make changes for DK or OK sprites (or that's the feeling I have, as I don't find any solution).

I know the PSP version is weird but as I said in this post I think it could be a way to make it.
The problem still lies in finding where OK and DK are in the code of the game. But if someone has figure it out, the easy solution would be to change everything (spritsheet and icons) in an unused sprite.
I need to ask this question as I have an issue with my actual project and I need a way to 1-Make another job... (not possible I guess) or 2-swap everything (stats, reactions...) between an obtainable job and OK or DK... and then swap it again for Squire (I don't care if that job can't use other skills like Onion Knights).
Do you know any fix to my problem?

Secondarily, I need an answer for this: It's weird but jobs like Archer or Ninja can't use their skillset away from their job?

Edit: It looks like there is not an easy fix due to the PSP mod limitations.